Intervertebral disk prosthesis

ABSTRACT

An intervertebral disk prosthesis designed to replace at least partially an intervertebral disk which is composed of a left half-disk, and a right half-disk, each of the half-disks covering a half-disk surface. The prosthesis is at least one prosthetic member each of which having a rigid upper plate, a rigid lower plate, and an elastic cushion placed between the upper plate and the lower plate. The elastic cushion has an upper face attached to the upper plate and a lower face attached to the lower plate. Each prosthetic member covers no more than the half-disk surface so as to replace either or both of the left half-disk and the right half-disk.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an internal prosthesis for human use designedto replace a deficient intervertebral disk in the lower back, andparticularly in the lumbar and lumbar-sacral regions.

2. Description of the Related Art

The general basic concern in osteoarticular surgery is to repair lesionsand possibly to prevent their aggravation, in order to preserve the twofunctions (the support function and the movement function) ofosteo-articular structures whenever possible.

The most frequent reason for surgery on the spinal column at the presenttime is mechanical deterioration of intervertebral support structures,the most important of which is the intervertebral disk.

Evolution of this deterioration usually has two harmful consequences:

firstly pain, and limitations of movement capabilities in the spinalcolumn itself;

secondly, deterioration of neurological structures contained in thespinal canal by mechanical compression, which causes peripheraldeficiencies.

Vertebral surgery therefore attempts to satisfy three objectives inthese cases:

1) restore the support function of the intervertebral structures;

2) restore the movement function of these structures;

3) release neurological structures for which the integrity may have beenadversely affected.

With current surgical techniques, it is impossible to satisfy thesethree objectives using a single operating procedure, due to the specificanatomic configuration of the spinal column.

firstly, it is usually only possible to satisfactorily treatneurological lesions by a posterior surgical approach, in which caseonly palliative actions can be done on the intervertebral disk whichwill remain deficient, or the disk can be completely eliminated byfusing the vertebra by bone grafts which will provide support but whichwill permanently eliminate movement;

secondly, and up to this day, the only way of replacing theintervertebral disk by a mobile prosthesis is to use an anteriorsurgical approach in which it is usually impossible to satisfactorilytreat posterior neurological lesions mentioned above at the same time.

Disk prostheses known at the present time provide total and single-piecedisk replacement, which is technically only possible by an anteriorapproach.

Furthermore, current disk prostheses restrict movements and do not haveall mechanical properties necessary to enable all desireable mobilitiesin the three planes in space and their combinations necessary forphysiological functioning of an intervertebral stage.

The concept of making prostheses, for example disk prostheses containinga flexible insertion part placed between two rigid disks (patentFR-1,122,634) was introduced a few decades ago.

Furthermore, in order to avoid these disadvantages, Dr. Arthur D.STEFFEE, M.D., designed a prosthesis which had a mechanical structurecapable of restoring support and all physiological disk movements at thesame time (EP-0.392.076). This was a remarkable improvement.

However once again, this equipment can only be installed by an anteriorsurgical approach and is only designed for complete replacement of adisk as a single part. To our knowledge at the present time, thisproduct has not been made available to surgeons.

SUMMARY OF THE INVENTION

The prosthesis according to the invention can remedy thesedisadvantages. Its appropriate structure, shape and dimensions aredesigned so that a half-disk can be replaced by a prosthetic part.

This makes it possible to achieve the three objectives mentioned above(restore the support function, restore the movement function and releaseneurological structures) in a single posterior surgical approach and ina single operation, possibly using two symmetrical prostheses in orderto replace an entire disk.

Consequently, the invention relates to an intervertebral disk prosthesisdesigned to be placed in the disk space containing an upper rigid plate,a lower rigid plate, an elastic cushion placed between the upper plateand the lower plate, and comprising an upper face and a lower face, eachbeing fixed to the corresponding plate.

The intervertebral disk prosthesis according to the invention comprisesat least one prosthetic member, constituting a fraction of a disk,corresponding at most to half a disk.

In a preferred embodiment, it is formed of two disk fractions eachforming a half-prosthesis.

This prosthesis can be inserted by a posterior approach through thespinal canal, which is a major advantage of the invention in surgicalpractice.

There is no mechanical disadvantage in using two prosthetic half-diskswhen a disk is to be replaced provided that dimensions are judiciouslychosen, and precise positioning will enable the two contiguoushalf-prostheses to work synchronously, forming the mechanical equivalentof a single piece disk unit.

Furthermore in some cases, use of a single half-prosthesis may bepreferred, conserving the other half of the natural disk if itscondition is still considered to be compatible with satisfactoryfunction which, in any case, will be assisted by the half-prosthesis.

When this seems feasible, the operation can thus be simplified.

This type of partial disk replacement may be indicated particularly:

in case of asymmetric deterioration of a disk, with the disk beingcompressed on one side;

in some scoliotic deformations in which the judicious use of ahalf-prosthesis at one or more than one level could rebalance thevertebral curvature while maintaining function.

It has the following useful characteristics, possibly combined dependingon which combinations are technically possible:

its global parallelepiped shape and appropriate dimensions enablesurgical insertion by posterior approach through the spinal canal;

the plates are made of a biocompatible material and are approximatelyrectangular;

each plate has a ledge type relief on its external face, preciselyoriented so as to prevent displacement of the prosthesis along aspecific direction after its installation;

the external surface of the plates between the ledge patterns is roughand has sharp edges to enable bone rehabitation from the vertebral bonesurfaces against which it is applied during surgical implantation;

the edges of the plates are rounded at each side, and within thethickness;

extraction notches are formed in the plates;

the cushion is a laminated composite elastic material, or is made ofsilicone or a product in the polyolefines family.

Complete replacement of a disk is not systematically necessary dependingon each pathological case encountered, but when it is necessary it willbe facilitated by symmetric insertion of two half-disk prostheses at thestage being operated on, one on the right side and the other on the leftside of the intervertebral space considered.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of the invention will be made with reference tothe drawings in the appendix, in which:

FIG. 1 shows a 3/4 perspective overall view of the front of a prosthesisaccording to the invention;

FIG. 2 shows a sagittal section through the prosthesis in positionbetween two vertebral bodies.

FIG. 3 shows an arrangement of two of the devices in position betweentwo vertebral bodies.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Posterior bone, ligament and neurological structures are omitted on FIG.2.

The upper metal plate 1 and lower metal plate 2 are composed of abiocompatible metal already tested in the field of orthopedic surgery.

Their radio-opaqueness is a major advantage for post-operationmonitoring of the position and behavior of the prosthesis.

In one specific embodiment, they may be made of titanium which isparticularly useful in that this material is radio-opaque and is alsocompatible with carrying out nuclear magnetic resonance examinations,which is very useful if post-operational endo-spinal canal explorationsare necessary.

It is beneficial if the peripheral surfacing of these metal plates iscast as a single piece. This provides two advantages:

surface irregularities 4 encourage rehabitation by bone tissue;

ledge shaped relief structures 5, which we will call "anti-withdrawalledges", which are oriented so as to prevent secondary migration of theprosthesis from the front towards the back, in other words from theintervertebral space towards the vertebral canal and the neurologicalstructures contained in it.

These two surface properties of the metal plates contribute to thestability of the prosthesis when in position, firstly by an immediategrip during installation, and secondly eventually through bonerehabitation.

The drawings in the appendix show three sets of two ledges on each metalplate distributed over the surface of the metal plate, but this exampleis not restrictive.

However, the line at which the first ledges are positioned is at adistance d from the posterior edge of the metal plate allowing the useof a specific supporting device used for initial positioning of theprosthesis between vertebral bodies. A distance d=7 mm is satisfactory.

The plates can also be composed, for example, of a biocompatiblecarbon-graphite type of composite material with the same morphologicalcharacteristics.

Furthermore, rigid plates 1, 2 have rounded edges 11, 21 and roundedcorners 12, 22 (foam) at the front in order to limit the risk oftraumatism of nearby anatomic structures when placing the prosthesis;this risk would be much too high if the corners and edges were sharp.

The rear surfaces 15 and 25 are straight, to facilitate inserting theprosthesis using the supporting device that bears on these surfaces.

Notches, called extraction notches 6, are formed in the metal of eachrigid plate, towards the posterior part of its edges.

These notches 6 are used to exert a backwards tension on the prosthesisusing an appropriate instrument, if another surgical operation isnecessary at a later date in which ablation of the prosthesis isnecessary.

The elastic cushion 3 is formed of a disk material with mechanicalproperties as similar as possible to the properties of a natural disk,in other words it must satisfy the requirements of physiologicalstresses with elasticity and strength properties as a function of thecompression, tension and shear forces, both in rotation and in sliding.

These mechanical properties have been defined using a great deal ofexperimental data determined from well known and reputable biomechanicalwork done on human anatomy parts.

The result is that the prosthetic disk material is made either of asilicone type elastomer, or of a laminated composite material or of anelastic material in the polyolefines family.

The prosthesis according to the invention is designed to correct thedeficiencies of an intervertebral disk in as many pathologicalconfigurations as possible.

It will preferably be used in the lumbar part of the spinal canal andthe design of the prosthesis is such that it can be placed at thislumbar level preferably by a posterior approach through the spinalcanal, this type of approach making it possible to treat posteriorlesions in the spinal column and to do a disk replacement in, a singlesurgical operation.

Each prosthesis according to the invention is designed to replace halfof a lumbar disk, from the right or from the left it replaces a fractionof a disk covering not more than half of the surface of the disk space.

If necessary, and depending on disk lesions diagnosed and then observedduring the operation, the two prostheses can be implanted, one from theright and one from the left in the same intervertebral stage, so as toperform a complete disk replacement, as shown in FIG. 3.

This placement using a posterior approach requires sufficient wideningof the bone canal to give a good exposure area and to provide goodprotection of nearby neurological structures.

The prosthesis according to the invention is inserted between thevertebral bodies after appropriate discectomy by preparing a sitebetween vertebra by recessing the vertebral plates using a specialinstrument.

FIG. 2 shows a sagittal section of a prosthesis according to theinvention in position between two vertebral bodies 7 and 8 with itsattachment devices 4, 5, at the prepared vertebral bone surfaces 9, 10,and shows the desirable preservation of an anterior peripheral part ofthe disk annulus 13, with an anterior-posterior thickness of the orderof 2 to 4 mm.

Furthermore, the intersomatic recess and the dimensions of theprosthesis are calculated as a function of the anatomic characteristicsof the case being operated, in order to create a safety space 14 of atleast 3 mm between the vertical line in the vertebral canal (posterioredge of vertebral bodies) and the posterior part of the prosthesis inposition.

This ensures that the prosthetic equipment does not project into thecanal.

A wide range of sizes of the prosthesis according to the invention areprovided with variations in all three dimensions, so that saidprosthesis can be used in different pathological cases, for differentanatomic configurations and at different lumbar intervertebral levels.

We have created the following variations as a result of practicaloperating experience in placing intersomatic grafts using the samemethod, but these examples are not restrictive:

it is designed with two different depths Pf; one equal to 25 mm and theother equal to 30 mm;

in each of these two depth categories, there will be variations in theheight H and width L of the prosthesis;

for each of the two depths considered, the height H may vary from 9 to13 millimeters, in steps of one millimeter;

for each defined depth and each height, the width L may vary from 9 to12 millimeters, in steps of one min.

The advantages of using the prosthesis according to the invention forthe dorsal spinal column may be very limited for two reasons:

firstly, due to the lack of natural physiological mobility of thisthoracic segment;

secondly, due to difficulties and dangers inherent to the presence ofthe spinal chord which is no longer present at the lumbar level.

However, it may be useful and it is possible to use the prosthesisaccording to the invention for the lower dorsal segments D11-D12 andD12-L1, for which restoration of mobility may be useful in somepathological cases.

Reference signs inserted after the technical characteristics mentionedin the claims are given only to facilitate understanding of the claims,and in no way restrict the scope.

I claim:
 1. Intervertebral disk prosthesis designed to replace at leastpartially an intervertebral disk which is composed of a left half-diskand a right half-disk, each of said half-disks being sized to cover ahalf disk surface, said prosthesis including at least one prostheticmember, each of said at least one prosthetic member comprising a rigidupper plate, a rigid lower plate, an elastic cushion placed between theupper plate and the lower plate and containing an upper face attached tosaid upper plate and a lower face attached to said lower plate, and eachof said at least one prosthetic member being sized to cover no more thansaid half-disk surface so as to replace one of said left half-disk andsaid right half-disk, wherein rear edges of said upper plate and saidlower plate are straight and remaining edges of the upper plate and thelower plate are rounded at each side, and within their thickness. 2.Intervertebral disk prosthesis according to claim 1, wherein saidprosthesis has a parallelepiped shape which enables surgical insertionby posterior approach through a spinal column.
 3. Intervertebral diskprosthesis according to claim 1, wherein said upper plate and said lowerplate are each substantially rectangular.
 4. Intervertebral diskprosthesis according to claim 1, wherein each of said upper plate andsaid lower plate comprises at least one wedge shaped structure (5) on anexternal face oriented to prevent movement of the prosthesis in aspecific direction after installation of said prosthesis. 5.Intervertebral disk prosthesis according to claim 4, wherein theexternal face (4) of each of said upper plate and said lower plate (1,2) surrounding said at least one wedge shaped structure (5) is rough andhas sharp edges facilitating bone rehabitation from vertebral bonesurfaces against which said external faces are applied during thesurgical implantation.
 6. Intervertebral disk prosthesis according toclaim 1, wherein said upper plate and said lower plate compriseextraction notches (6).
 7. Intervertebral disk prosthesis according toclaim 1, wherein the elastic cushion (3) is made of a laminatedcomposite elastic material.
 8. Intervertebral disk prosthesis accordingto claim 1, wherein the cushion (3) is made of silicone or a product inthe polyolefins family.
 9. Intervertebral disk prosthesis according toclaim 1, wherein said prosthesis is formed of two prosthetic members,one of said two prosthetic members corresponding to said left half-diskand another of said two prosthetic members corresponding to said righthalf-disk.